The rules governing image formation of thin specimens in scanning and scanning transmission electron microscopy at low energy, deduced from the observation of semiconductor multilayers, were validated on specimens defined by a much more complex structure as the biological ones. It is shown that for a suitable specimen thickness it is possible to have, at the same time, backscattered electron images and scanning transmission electron images with a comparable resolution. Moreover, the nonconductive biological samples can be observed without charging effects if they are thin enough to ensure that a significant fraction of the electron beam crosses the specimen.